The present invention relates to a cover sheet which covers the rotatable means of adjustable electronic devices such as potentiometers or the like. Conventionally, such electrical devices include rotor means and stator means contained in a case. The rotor means are exposed and therefore require special handling precautions during the manufacturing process. Examples of such a rotary electric device are a trimmer capacitor, a variable resistors, variable coils and rotary switches. The present invention may be applied to any type of such rotary electric devices.
A rotary electric device comprises a rotary mechanism portion including a rotor means and a stator means contained in a case. In such a rotary electric device, the rotor means is rotated to change, e.g., the capacity in a trimmer capacitor or the resistance value in a variable resistor. Therefore, the case of the rotary electric device is provided with an opening for performing desired adjustment by changing characteristics of the rotary electric device from the exterior thereof, wherefore the rotor means is rotated through the opening.
Generally when a rotary electric device is mounted on a printed circuit board etc., flux employed in soldering may enter the case through the opening formed therein and this produces undesirable effects which alter the characteristics of the rotary electric device. Further, for improving workability and reducing production costs, it is desired to pass all of electric or electronic components through a molten solder tank thereby to simultaneously solder a plurality of electric or electronic components on the printed circuit board etc. In this case, however, the solder inevitably enters the rotary electric device through the opening formed in the case to disadvantageously influence the rotor means and stator means etc., and as the result, for example, rotation of the rotor means might be prevented and the response characteristics of a dielectric member or resistance film serving as the stator means might be degraded. Thus, the soldering of a rotary electric device such as a trimmer capacitor always had to be carried out separately and after the printed circuit board has gone through the solder tank.
In this regard, there has been provided the so-called closed type rotary electric device to which a cover sheet is applied to close an opening defined in its case thereby to prevent entry of flux and solder during the assembly thereof. Such a closed type rotary electric device is disclosed in, for example, Japanese patent laying-open gazette No. 72353/1983, published on Apr. 30, 1983. In this document, a heat-resistant cover sheet of polyimide resin is disclosed to be applied to the outer surface of the case to close an opening defined therein by a heat-resistant adhesive. In the closed type rotary electric device, therefore, entry of flux or solder is prevented during the different production and assembly phases of the device. This enables simultaneous soldering of the rotary electric device with other electric or electronic components.
The cover sheet is made of material having such thickness and strength that it can be easily broken by a screwdriver etc., and hence the rotor means can be readily rotated by the screwdriver etc. with the cover sheet being broken by the same when it is necessary to rotate the rotor means of the rotary electric device.
However, in the aforementioned arrangement, it has been difficult to apply the cover sheet to the case of the rotary electric device. When a plurality of cover sheets are applied one by one to small-sized rotary electric devices of, e.g., several millimeters square, the sizes of the rotary electric devices and the cover sheets are too small to process, and hence it is extremely difficult to apply such cover sheets to the small-sized rotary electric devices. Although, it is possible to prepare a large-sized cover sheet member and apply it to a plurality of rotary electric devices, the large-sized cover sheet member must be cut after application to the devices to provide an individual cover for each device. It is troublesome to cut the cover sheet member along the outer peripheries of cases of the rotary electric devices. Further, the thickness or the height of the rotary electric device is increased by application of the cover sheet, and such increase in size is not desirable particularly in a small-sized rotary electric device. Further, when a cover sheet made of polyimide resin is broken as the rotor is rotated, the fragments of the cover sheet may enter the case.